1. Field of the Invention
The present invention relates to a throttle device for a multipurpose engine, and more particularly relates to a throttle device for a multipurpose engine that is configured such that a throttle valve opens and closes with the aid of an actuator.
2. Description of the Related Art
In multipurpose engines such as spark ignition internal combustion engines used as drive sources for electrical generators, farming machinery, and various other applications, the opening of the throttle valve is usually adjusted by a mechanical governor made up of a weight and spring to control the engine speed.
A technique has recently been proposed for the precision control of the engine speed using an electronically controlled throttle device (i.e., an electronic governor) for opening and closing the throttle valve in this type of multipurpose engine with the aid of a stepping motor or other actuator.
Operation of the multipurpose engine is usually stopped by discontinuing or cutting the ignition. The throttle opening of the multipurpose engine at this time is held fully open or half open in view of the structural features of the mechanical governor or in order to enhance the restarting properties of the engine. However, since most multipurpose engines use a carburetor as a fuel supply device, when the throttle valve is held open when operation is stopped, the supply of fuel is not immediately stopped, and dieseling or run-on can occur.
Therefore, the conventional design has included a fuel cut valve (solenoid valve) disposed along the fuel channel of the carburetor, and the fuel cut valve is closed such that the supply of fuel is blocked simultaneously with the discontinuing of ignition, as taught in Japanese Laid-Open Patent Application No. 2003-120422, for example.
However, when a fuel cut valve is disposed along the fuel channel of the carburetor as described above, drawbacks arise whereby the structure of the carburetor becomes complex, which leads to increased size and cost of the carburetor.
Aside from the above, in an electronically controlled throttle device, it is generally the case that a valve shaft for carrying the throttle valve is connected with an actuator by a speed reduction gear mechanism, and the output of the actuator is transmitted to the throttle valve. The throttle valve is adapted to be urged in or towards the closed position by a return spring or other urging member, as taught in Japanese Laid-Open Patent Application Nos. 2001-263098 (paragraph 0008 and others) and 2003-83093 (paragraph 0007 and others).
When the throttle valve is to be opened from a fully closed position, since the throttle valve is urged in or towards the closed position, it takes a time until the throttle valve is actually opened due to the backlash effect of the speed reduction gear mechanism, disadvantageously rendering response to be inadequate. This will be the same even when no urging force is applied in the closed position.
Further, a torque, more precisely pressure in the valve closing direction also acts on the throttle valve due to the flow of intake air. A drive force that is larger than the combined force of the urging force of the urging member and the pressure of the intake air is therefore required in order to open the throttle valve, thus creating the drawback of having to increase the size of the actuator for opening and closing the throttle valve.